UK gardening help and assistance


Green Manure Crops

Green Manure is a crop sown purely for the purpose of digging it in while it is still fresh and green to increase the fertility of the soil. Various crops are suitable, the commonest being mustard, rye, spinach and tares. If it is desired to add nitrogen in large quantities, then peas, tares, clover, or lupins are sometimes used. It is important to dig the crops in before they get to the flowering stage. They not only add humus to the soil, but help to retain the soluble plant foods which were present when the crop was sown. Such foods may easily be washed away during a rainy period.

Besides recycling nutrients in the soil for plant uptake, green manure crops used in rotation with other crops help reduce plant diseases and insects in the garden. Organic matter also helps improve the water holding capacity of sandy soils while aerating clay soils.

Directly the ground is free of a productive crop, one of these green manure plants can be sown instead of leaving the land bare. Spinach and rye are often sown for green manure in September or October; mustard and tares are often sown in April. Mustard has the advantage that it may be dug in less than eight weeks after sowing; but it has the disadvantage that it is liable to get club root, and so keep the disease going in the ground.

It often helps to topdress green manure crops with a little livestock manure or nitrogen fertiliser to help them get started. The crop may go dormant in the winter but will remain relatively green. This helps stop the soil being blown or washed away by wind or rain.

Turn green manure crops under by digging or using a rotavator about 4 to 6 weeks before planting the summer garden. A little extra nitrogen fertiliser will help micro-organisms in the soil break down plant material into humus. Since nutrients in the humus are in organic forms, they are released slowly to plants over time.

It is a good practice to have some suitable "green manure" seed available at all times so that directly a patch of ground is cleared of crops, and is not required definitely for anything else, the green manure crop can be sown and the resultant crop dug in. Green manure crops are a good substitute for farmyard manure which the gardener can incorporate quite easily.

Green Manure

Sow Growing period Suitable soil types Nitrogen fixer Other info
Winter tares Jul - Sep up to Oct, or over winter heavy, not too acid yes hardy
Mustard Mar - Sep 2 - 8 weeks moist, fertile no can suffer clubroot
Hungarian grazing rye Aug - Nov autumn - spring most >no hardy
Alsike clover Apr - Aug few months to 2 years damp, acid yes short-term perennial
Field beans Sep - Nov over winter damp, heavy yes turn under before flowering
Buckwheat Apr - Aug 2 - 3 months tolerates poor no not hardy
Phacelia Aug autumn - spring not too heavy no not hardy everywhere
Red clover Early Aug anytime, useful long term avoid acid, add lime yes sow thickly, cut and mulch regularly